Early replicating DNA of Physarum is denser than late replicating DNA

Flow cytometry reveals a high degree of genomic size variation and mixoploidy in various strains of the acellular slime mold Physarum polycephalum

High-resolution flow cytometry, using avian erythrocytes as an internal standard, was employed to study constitutive genome size variation of G2-phase nuclei of Physarum polycephalum strains during the macroplasmodial stage of their life cycle. Our results document a previously unknown extent of genome size variation and mixoploidy in this organism. The unimodal diploid strain Tu 291 displayed the largest genome of the strains tested; in contrast, the Colonia strain displayed only half of the Tu 291 G2-phase fluorescence, confirming its haploid nature. An additional strain, derived from a recent cross between Lu897 and Lu898 amoebae, must have arisen by selfing (propagation of only one of the parental genomes to the macroplasmodial stage), since its nuclei display close to the haploid G2-phase DNA content. The observation of a small fraction of corresponding diploid nuclei within the haploid population of this strain, while maintained as microplasmodia, supports the notion that meiosis in haploid strains may require the presence of diploid nuclei. Two of the descendants of the prototype haploid Colonia strain, which were kept for extended periods of time in submerse culture, proved to be near diploid and mixoploid. Polyploidization and subsequent loss of DNA thus seems to contribute to the extremes of genome size variation in Physarum. In addition to unimodal fluorescence distributions, a number of diploid strains displayed bi- and even trimodal distributions within harvests of a single G2-phase macroplasmodium. Analysis of these mixoploid strains by means of gaussian curve-fitting suggests that the smaller genome size differences in Physarum may arise in step-wise diminution of DNA in approximate units of 3-5% of the original Tu 291 genome.

Mitotic cell cycle control in Physarum. Unprecedented insights via flow-cytometry

High resolution flow-cytometric studies of isolated macroplasmodial nuclei of the myxomycete. Physarum polycephalum provide definite evidence for the persistence of natural synchrony at mitosis throughout the entire cell cycle, i.e. completely synchronous DNA replication and traverse of G2. Even if nuclei within a given macroplasmodium belong to two distinct genome size classes (mixoploidy), they cycle and traverse mitosis in strict synchrony. This cannot be explained by current models of regulation of division based solely upon nuclear size and/or nuclear/cytoplasm ratios. Constitutional DNA content variation was apparent among all tested strains, and loss of late-replicating, presumably AT-rich DNA accounts for this variation. A constant duration of the S phase is maintained, irrespective of DNA content, via differential slowdown of replication rates during the 2nd and 3rd hours of replication. A frequently described extension of nuclear replication into G2 could not be substantiated. Interference with DNA and protein synthesis provides the first evidence for a brief "G1 phase" equivalent of 3-4 min duration in asynchronous microplasmodial cultures, and temporally assigns a protein synthesis-dependent "transition point" for completion of mitosis and initiation of DNA synthesis at 5 min prior to actual division nuclei which have passed this point at the time of addition of cycloheximide replicate 5% of their DNA before they become arrested. These findings provide strong experimental support for the transition point concept of cell cycle control, and additionally are commensurate with some form of the replicon-set hypothesis in Physarum.